The present invention relates to an excited frame conveyor and more specifically to conveyors which utilize vibratory motion for the distribution, collection, or transfer of a product.
Vibratory conveying systems are commonly used in the food and other industries where belt type conveyors are undesirable or where it is difficult to maintain the conveyors in a sanitary condition. Vibratory conveyors make use of a conveyor bed that includes an elongated tray which is made to vibrate predominantly in a desired direction and at an angle such that materials deposited on the bed will migrate or travel at a selected speed from an infeed end of the bed to a discharge end.
Excited frame vibratory conveying devices are known in the art. For example, U.S. Pat. No. 4,313,535, which is incorporated by reference herein, shows an exemplary excited frame conveying apparatus that has performed well in the past for transporting bulk product over relatively short distances.
As best seen in FIGS. 1 and 2, a prior art excited frame conveyor similar to that shown in the patent above, includes a vibratory drive mounted on an elongated frame. The frame is supported between a rigid, ground supported base, and the bed of the conveyor receives and moves product along the conveyor length. A resilient suspension in the form of leaf or beam springs project upwardly at spaced intervals along the frame, and are inclined in the direction of the intake or infeed end of the conveyor.
The elongated conveyor or product transporting bed is mounted on the upper ends of the respective beam springs. The conveyor bed is supported by the beam springs in a generally parallel relationship to the frame, and in a substantially overall horizontal orientation. Due to the resiliency of the respective beam springs, the product conveying bed is capable of moving relative to the frame in response to a force supplied to the bed by the vibratory drive mounted on the frame. During operation of the apparatus, the vibratory drive produces an oscillating vibratory force. This force may be generated by rotating eccentric weights mounted on the vibratory drive.
As will be recognized the vibratory drive is mounted on the frame and therefor imparts vibratory motion to the frame, which is then transferred through the beam springs to the conveyor bed. As a result, the bed vibrates at substantially the same frequency as the drive and frame.
Isolation springs are also typically mounted between the frame and the earth""s surface by way of supports that are spaced along the length of the conveyor. The isolation springs are used to xe2x80x9cisolatexe2x80x9d vibration of the frame to minimize transmission of vibratory forces to the supports.
A conveyor bed that is displaced from its xe2x80x9cat restxe2x80x9d position and then allowed to oscillate freely will oscillate at its natural or xe2x80x9charmonic frequencyxe2x80x9d. This frequency is dependent upon the combined spring constant, the number of springs supporting the bed, as well as the mass of the bed.
As a general matter for conveyors of this design, less vibration and force is transferred to the floor or other supporting structure by an excited frame conveyor design because of the small vibrational amplitude of the frame, as compared to the vibrational amplitude of the product conveying bed. The low level of vibrational force transferred to the surrounding structure is a chief advantage of the prior art excited frame vibratory conveyor as seen in FIGS. 1 and 2. In view of the relatively short length and rigidity of the frame operational frequencies of the conveyor bed do not generally approach the frequency modes in the frame at which the frame and conveyor bed will begin to move in directions not conducive to transport of product.
It should be understood that it is not unusual for a conventional excited frame conveyor to operate at frequencies over the natural frequencies of the system (system frequencies). An example of such a device is illustrated in the O""Connor patent (U.S. Pat. No. 2,353,492). In this reference the beds and frames are considered to be rigid members (due to the typically short machine length and the rigid mass of the structures). As such, the structural frequencies of the bed and frame are much higher than the operating frequencies. The frequencies of interest in O""Connor are actually isolator frequencies which relate to the springs and masses (as rigid members) only. Conventional drives are easily capable of ramping or accelerating through these isolator frequencies in this type of system, and may operate at frequencies which do not cause the bed or frame structural frequencies to become a design consideration.
It has long been recognized that excited frame vibratory conveyors under certain operational conditions may excite frame structural frequency modes that result in undesirable and even destructive motion of the conveyor bed, or the entire conveyor system. As a general matter this is usually not a problem with the above-noted short frame conveyors where the frame frequencies are well above the operating frequencies. However, if longer frames are fabricated, the structural natural frequencies of the frame decrease and become important factors in the overall conveyor design.
To overcome the problems encountered when conveyor lengths increase, the frame must be stiffened in order to keep the frame structural frequencies well beyond the desired operational frequencies. This solution however does not remain commercially practical beyond a length of approximately 40 feet, due to cost constraints. Thus, when it becomes desirable to transport materials on an excited frame conveyor over distances greater than 40 feet, serious consideration must be given to the issue outlined above.
As would be expected an excited frame will become xe2x80x9csoftxe2x80x9d or will otherwise bend more readily as the length of the frame increases. Along with increasing frame length, the distance decreases between frequency modes at which the frame may become excited and begin to move in undesirable directions. Still further, as frame length increases, the frequencies at which undesirable modes occur decrease. This relationship is shown graphically in FIG. 7 of the drawings, where three separate frame structural modes are shown and which decrease in frequency with a corresponding increase in the frame length. Still further the distance between modes correspondingly decrease as the frame length increases. The techniques for designing conveyors less than 200 inches are well known in the art, especially since structural modes do not typically come into play at frame lengths under that length.
If a long conveyor is to be operated below its structural frequency modes to avoid the difficulties noted above, the obvious solution is to lower the operating frequency of the drive below the undesired structural frequency or increase the stiffness of the frame. However, lowering the operating frequency decreases the conveying speed which reduces the capacity of the conveyor; while increasing the frame stiffness can significantly increase the cost.
Experience has shown that excited frame conveyors should move material at flow rates of approximately 20-150 feet per minute. To achieve such flow rates with conveyors greater than 40 feet, the operating frequency must typically be beyond at least the first structural frequency of the conveyor frame, unless the frame is braced or otherwise made sufficiently rigid such that the frequency modes occur beyond the operating frequency.
In addition to the foregoing, the energy required to maintain a desired flow rate must also increase because of the added mass of the bed. As a result, a larger drive will be required which adds significant weight to the conveyor and adds complexity to the frame. Therefore, long excited frame conveyors have heretofore been thought to be uneconomical. At least a part of the problem with long xe2x80x9csoftxe2x80x9d frames (those that can bend) is seen during start-up of the conveyor drive. As noted above, the drive is typically an eccentric mass vibratory drive (typically a motor with eccentric weights mounted to a rotatable drive shaft) that must xe2x80x9cramp upxe2x80x9d or accelerate from a speed of zero to an operating speed (RPM) where the frame will vibrate at a predetermined frequency that is beyond at least the first structural frequency of the frame. It is very difficult and often impossible for a conventional drive to pull or take the frame through the first structural frequency.
As noted previously, if a long excited frame conveyor is to operate at normal speeds (to produce the 20-150 feet per minute flow rates experienced with shorter frame conveyors), the vibrational frequency produced by the eccentric mass of drive should be above the first structural frequency of the frame. Thus the drive must ramp up or move through at least the first structural frequency of the frame to reach operating speeds. As the drive ramps up to an operational speed, the conveyor components are exposed to vibratory forces that build in amplitude and vary in direction until a given design point is reached at an operational speed (RPM) where the conveyor functions in a desired manner. Undesired excitation of the frame structural frequency modes should be avoided or minimized when a drive is xe2x80x9cramping upxe2x80x9d to operational speed so that maximum drive energy is focused in the desired direction.
As will be recognized, energy that produces undesired frame motion is squandered if it is not used to move product. More importantly, the inertia that is built-up in undesired directions becomes difficult, and may be impossible under some circumstances to overcome with a conventional drive motor. That is to say, the motion and inertia of a laterally oscillating conveyor bed cannot be easily changed into longitudinal oscillating motion because the inertia built-up in the lateral direction must first be overcome. Therefore, the torque required to drive a long frame conveyor through a structural frequency becomes increasingly expensive.
It also becomes increasingly difficult to drive or move through frame structural frequency modes as the drive approaches a given operational speed. This is due to the inertia of the vibrating components. For example, if a frame exhibits an undesirable lateral movement component at low RPM minimal inertia will develop in the frame. As such, the drive could conceivably have ample torque to drive or move the frame through that particular mode. However, as inertia of the frame increases with increasing drive RPM, and when a frame structural frequency mode is encountered near the operational RPM of the drive, significant additional energy (torque) will need to be supplied to overcome the increased inertia developed in the undesired direction. In fact, it has been known that a vibratory drive may stall in a frame natural frequency mode for lack of sufficient energy to overcome the inertia accumulated in the undesired direction.
Prior forms of excited frame conveyors such as that disclosed by U.S. Pat. No. 4,313,535 have drive systems designed such that the primary driving forces lie along a line similar to that shown in FIG. 2, which passes through the system center of mass. The force line is typically at an acute angle to the long dimension of the frame and bed. When it becomes desirable to design a conveyor frame of a length where frame flexibility enters as a design consideration, such angular forces become problematical. Oscillational forces applied at an angle to a long, flexible excited frame would tend to bend the longer xe2x80x9csoftxe2x80x9d frame during operation. Therefore, long excited frame conveyors with a drive applying a force angularly through the center of mass has simply not been considered a viable possibility.
In view of the foregoing, the present invention is directed to an excited frame conveyor wherein the drive limits the amount of energy put into the frame of an excited frame conveyor as the drive xe2x80x9cramps upxe2x80x9d to operating frequency. Still further, minimal damage or wear to the conveyor is avoided as the frame is moved through problematic frequencies. Yet further, increased conveyor lengths are achieved without requiring a corresponding increase in frame stiffness.